1. Field of the Invention
The present invention relates to a polyimide resin, a polyamic acid which gives such a polyimide resin, a circuit board comprising a polyimide resin, and a wireless suspension board. More particularly, the present invention relates to a novel polyimide resin having a low linear/expansion coefficient and a low dielectric constant which can be used as various protective layers or interlayer insulation layers in semiconductor, oriented layers in liquid crystal element, base films in flexible printed-wiring board, gas-permeable membranes, heat-resistant adhesives, etc., and a polyamic acid which gives such a polyimide resin.
The present invention further relates to a circuit board comprising an insulation layer comprising a polyimide resin provided on a metal foil substrate and a wireless suspension board comprising a pattern circuit comprising a conductor layer provided on such a circuit board.
In order that a magnetic disc device such as hard disc device used as an external memory device for computer, etc. performs magnetic recording or reproduction, it is necessary that the magnetic disc and the magnetic head be allowed to run relative to each other to produce an air stream against which the magnetic disc is elastically pressed onto the magnetic disc so that a predetermined minute clearance is kept therebetween. The magnetic head supporting device for elastically pressing the magnetic head onto the magnetic disc against the air stream is a suspension. The present invention relates to a circuit board which can be preferably used to produce such a wireless suspension board and a suspension board with patterned circuit comprising a circuit comprising a conductor layer formed on such a circuit board by patterning technique.
2. Description of the Related Art
A polyimide resin makes the use of its excellent mechanical strength or electrical insulation in addition to its high heat resistance to find wide application in electrical and electronic fields. However, polyimide resins which have heretofore been known normally have a poor solubility in an organic solvent. Accordingly, in order to obtain a polyimide resin film for example, a polyamic acid which is a precursor thereof is dissolved in an organic polar solvent to give a solution which is casted, and then heated to a temperature as high as 300xc2x0 C. or higher so that it is subjected to ring closure and imidation.
In the recent years, on the other hand, a polyimide resin having a small dielectric constant has been required to realize faster processing of signal. To this end, various polyimide fluoride resins having fluorine atom incorporated in molecules have been developed. However, such a polyimide fluoride resin which has heretofore been known normally has a high linear thermal expansion coefficient. As described in T. Matsushita et al., xe2x80x9cMacromoleculesxe2x80x9d, 26, 419, 1993, a polyimide fluoride resin obtained by the reaction of 2,2xe2x80x2-di(trifluoromethyl)-4,4xe2x80x2-diaminobiphenyl with 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane has a dielectric constant as low as 2.8 but has a linear thermal expansion coefficient as high as 45 ppm.
Referring particularly to circuit board, as a thin multi-layer circuit board for the purpose of enhancing the packaging density of semiconductor or the speed of processing of signal there has recently been used a circuit board comprising an insulation layer comprising a polyimide resin provided on a metal foil. However, since the polyimide resin which has heretofore been normally used as an insulating material has a greater linear thermal expansion coefficient than various metal foils, the resulting circuit board is liable to warping thereof or cracking or peeling of the resin layer.
In order to solve these problems, 2,2xe2x80x2-di(trifluoromethyl)-4,4xe2x80x2-diaminobiphenyl and pyromellitic anhydride are reacted to obtain a polyimide fluoride resin having a linear thermal expansion coefficient as low as 0 ppm. However, this polyimide fluoride resin has a dielectric constant as high as 3.2.
Further, computers and memories as its peripheral equipment have been required to have a greater capacity as well as a smaller size or lower cost. Under these demands, technology for hard disc drive has shown a remarkable progress among many technologies. Referring to magnetic head, it has been of urgent necessity to develop thin layer magnetic head (TFH) comprising a coil portion having a reduced thickness or even thin layer-magnetic resistor composite head (MR) which can both read and write and has a drastically great memory capacity instead of the metal-in-gap (MIG) type head of the related art.
However, depending on the technique of the related art which comprises drawing a conductor line on a suspension board to form a desired wiring, the conductor line can have an effect on the elastic modulus of the suspension, causing the change of the floated amount of magnetic head. In some cases, the magnetic head can come in contact with the magnetic disc, deteriorating the durability of the magnetic disc device.
Under these circumstances, a suspension comprising an electrical circuit formed directly on a suspension board having a head mounted thereon has recently been put to practical use. As previously mentioned, however, a suspension which is eventually obtained comprising the circuit board of the related art comprising a polyimide resin as an insulation layer, too, can be subject to defective insulation or warping leading to defective properties because the polyimide resin exhibits a greater linear thermal expansion coefficient than metal foil substrate.
The present invention has been worked out to solve the foregoing problems with the polyimide resin of the related art as a first aspect.
An object of the present invention is to provide a novel polyimide resin having a low linear thermal expansion coefficient and a low dielectric constant in addition to excellent characteristics inherently possessed by a polyimide resin and a polyamic acid which gives such a polyimide resin.
The present invention has also been worked out to solve the foregoing problems with the circuit board of the related art comprising an insulation layer comprising a polyimide resin provided on a metal foil and wireless suspension board comprising such a circuit board as a second aspect.
Another object of the present invention is to provide a circuit board which comprises an insulation layer comprising a polyimide resin having a low linear thermal expansion coefficient close to that of various metal foils and a low dielectric constant provided on a metal foil and thus is not liable to cracking or peeling of the resin layer as well as warping of the resin layer.
A further object of the present invention is to provide a wireless suspension board comprising such a circuit board.
The present invention provides a polyamic acid having a weight-average molecular weight of from 20,000 to 100,000 comprising a repeating unit represented by the following formula (I): 
wherein X represents a tetravalent organic group represented by the following formula (a): 
or the following formula (b): 
and Y represents a divalent organic group represented by the following formula (c): 
(in which Z represents a hydrogen atom or a fluorine atom), the following formula (d): 
(in which Z represents a hydrogen atom or a fluorine atom), the following formula (e): 
the following formula (f): 
or the following formula (g): 
wherein the organic group represented by the formula (a) and the organic group represented by the formula (b) account for from 20 to 99 mol % and from 1 to 80 mol % of the organic groups represented by X, respectively, and the organic group represented by the formula (C) and the organic group represented by the formula (d), (e), (f) or (g) account for from 20 to 99 mol % and from 1 to 80 mol % of the organic groups represented by Y, respectively.
The present invention also provides a polyimide resin having a weight-average molecular weight of from 20,000 to 100,000 comprising a repeating unit represented by the following formula (II): 
wherein X represents a tetravalent organic group represented by the following formula (a): 
or the following formula (b): 
and Y represents a divalent organic group represented by the following formula (c): 
(in which Z represents a hydrogen atom or a fluorine atom), the following formula (d): 
(in which Z represents a hydrogen atom or a fluorine atom), the following formula (e): 
the following formula (f): 
or the following formula (g): 
wherein the organic group represented by the formula (a) and the organic group represented by the formula (b) account for from 20 to 99 mol % and from 1 to 80 mol % of the organic groups represented by X, respectively, and the organic group represented by the formula (C) and the organic group represented by the formula (d), (e), (f) or (g) account for from 20 to 99 mol % and from 1 to 80 mol % of the organic groups represented by Y, respectively.
The present invention further provides a circuit board comprising an insulation layer comprising a polyimide resin provided on a metal foil substrate.
The present invention further provides a wireless suspension board comprising an insulation layer comprising a polyimide resin provided on a metal foil substrate and a pattern circuit comprising a conductor layer provided thereon.